Synergistic effect of dual-frequency ultrasound irradiation in the one-pot synthesis of 3,5-disubstituted isoxazoles.

Herein is reported a one-pot three-step process for the regioselective synthesis of 3,5-disubstituted isoxazoles based on copper(I)-catalyzed cycloaddition reaction between in situ generated nitrile oxides (from the corresponding aldehydes) and alkynes, using ultrasound irradiation, avoiding toxic reagents and solvents and isolation/purification of intermediates. The combined use of 40 kHz ultrasonic bath and 20 kHz probe in the presence of copper turnings reduced reaction time to 1h and resulted in only one final purification step with increased yields, clearly indicating that there is a dual-frequency synergistic effect. In addition, under metal free conditions, the 1,3-dipolar cycloaddition was regioselective giving low to modest yields.